This invention relates to projection lens systems for use in projection televisions and, in particular, to low cost, high performance projection lens systems for use in projection televisions that employ three cathode ray tubes (CRTs), e.g., a red CRT, a blue CRT, and a green CRT.
There exists a need in the art for projection lens systems and, in particular, rear projection lens systems, that have some and preferably all of the following features:
(1) The systems can be produced at low cost so as to be suitable for use in high volume consumer projection television sets.
(2) The systems can accommodate the spectral differences in the light produced by the red, green, and blue CRTs without the cost and complexity associated with full color correction.
(3) The systems have an optical performance suitable for use with the higher bandwidth signals of digital televisions.
(4) The systems exhibit a high level of image contrast.
(5) The systems produce a bright image, e.g., the systems have infinite conjugate f/#""s that are less than or equal to 1.5 and preferably are around 1.0.
(6) The systems have a wide field of view in the direction of the screen so that the distance to the screen can be reduced, e.g., a half field of view in the direction of the screen of at least 35xc2x0.
(7) The systems are relatively insensitive to changes in temperature, e.g., changes between room temperature and operating temperature.
To satisfy this need in the art, the invention provides projection lens systems which have some and preferably all of the above seven features.
In accordance with a first aspect, the invention provides a projection lens system for use in a projection television which has a screen and a first CRT which produces light of primarily a first color, a second CRT which produces light of primarily a second color, and a third CRT which produces light of primarily a third color, said projection lens system comprising three projection lenses, one projection lens being associated with each of the CRTs during use of the system for forming an image of the light produced by that CRT on the screen, each projection lens consisting of:
(A) a first lens unit (U1) on the long conjugate side of the lens, said first lens unit having a positive power; and
(B) a second lens unit (U2) which (i) is associated with a CRT during use of the lens, (ii) has a strong negative power when so associated, and (iii) provides most of the correction of the lens"" field curvature;
wherein in addition to any difference based on satisfying the Scheimpflug condition (see, for example, Hasegawa, U.S. Pat. No. 5,045,930; Yamamoto et al., U.S. Pat. No. 5,293,226; and Toide et al., U.S. Pat. No. 5,537,167) or any difference in spectral transmission (see for example, Wessling, U.S. Pat. No. 5,055,922), the second lens unit of the second projection lens differs from the second lens unit of the first projection lens in at least one optical property, said difference being based on said first and second colors.
In certain preferred embodiments, in addition to any differences based on satisfying the Scheimpflug condition or any differences in spectral transmission, the second lens units of the first, second, and third projection lenses all differ from one another in at least one optical property, said differences being based on the first, second, and third colors.
In other preferred embodiments, the second lens units comprise a meniscus element and the differences between second lens units are achieved through differences, other than spectral transmission, in the meniscus elements, e.g., differences in at least one of:
(1) focal length,
(2) index of refraction,
(3) base radius for the screen side surfaces of the elements,
(4) base radius for the CRT side surfaces of the elements,
(5) difference in surface shape and/or best-fit spherical radii of the screen side surfaces for elements that have aspherical screen side surfaces, and/or
(6) difference in surface shape and/or best-fit spherical radii of the CRT side surfaces for elements that have aspherical CRT side surfaces.
In accordance with other embodiments, the second lens units comprise a coupling fluid portion (e.g., a coupling fluid between a meniscus element and the faceplate of the CRT which in addition to optically coupling the lens to the CRT faceplate also functions as a cooling medium), and the differences between second lens units are achieved through differences, other than spectral transmission, in the optical properties of the coupling fluid portion of the second lens units. Such differences in the coupling fluid portion of the second lens units include differences in index of refraction of the coupling fluid produced through, for example, differences in composition and/or differences in coupling fluid temperature resulting from heating and/or cooling of one or more of the coupling fluids and/or its housing. Such differences also include differences in the shape (e.g., axial thickness and/or radii of curvature) of the coupling fluid portion. The differences in the coupling fluid portions of the second lens units can include both differences in index of refraction and differences in shape.
As another alternative for producing differences in second lens units, the optical properties, other than spectral transmission, of the faceplates of the CRTs, which form part of the second lens unit during use of the projection lens, can be made different for at least two of the CRTs, e.g., for the green and red CRTs. Such differences can include differences in thickness, index of refraction, and radii of curvature. Also in the case of CRT faceplates that include one or more aspherical surfaces, surface shape and/or best-fit spherical radii can also be made different between various of the faceplates based on color. In general, this approach of varying the faceplate is less preferred for manufacturing and cost reasons than varying the properties of a meniscus element (most preferred) or varying the properties of the coupling fluid region of the second lens unit.
In accordance with other preferred embodiments, the first lens units of the projection lenses are identical to within manufacturing tolerances. In this way, the manufacturing cost of the system can be reduced since common first lens units are used for the main, most complex part of the projection lenses while at the same time high levels of optical performance can be achieved by varying only a relatively small, simpler part (the second lens unit) of the projection lens for some or all of the different colors.
In accordance with a second aspect, the invention provides a projection lens for use in combination with a CRT and having a long conjugate side, a short conjugate side, and a focal length F0 when associated with the CRT, said lens consisting in order from its long conjugate side of:
(A) a positive first lens unit (U1) which consists in order from the lens"" long conjugate side of:
(i) a first lens subunit which consists of a first lens element (L1) which has at least one aspherical surface and a weak power;
(ii) a second lens subunit (focal length=F2) which is preferably biconvex and which provides most of the positive power of the projection lens and consists of a second lens element (L2) or a doublet (DB); and
(iii) a third lens subunit which consists of a third lens element (L3; focal length=F3) which has at least one aspherical surface and a positive power; and
(B) a second lens unit (U2; focal length=F4) which (i) is associated with the CRT during use of the lens, (ii) has a strong negative power when so associated, and (iii) provides most of the correction of the lens"" field curvature;
wherein:
(a) the first lens element has a best-fit spherical radius R11 in the direction of the lens"" long conjugate side and a best-fit spherical radius R12 in the direction of the lens"" short conjugate side;
(b) the second lens subunit has a radius R21 in the direction of the lens"" long conjugate side and a radius R22 in the direction of the lens"" short conjugate side;
(c) the second lens subunit is axially spaced from the third lens element by a distance T23;
(d) the third lens element has an axial thickness T3, a best-fit spherical radius R31 in the direction of the lens"" long conjugate side, and a best-fit spherical radius R32 in the direction of the lens"" short conjugate side; and
(e) the third lens element is axially spaced from the second lens unit by a distance T34;
and wherein the projection lens has some and preferably all of the following characteristics:
(i) |R22|/R21xe2x89xa71.5 (or xe2x89xa72.0 or xe2x89xa72.5);
(ii) R31 less than 0;
(iii) R32 less than 0;
(iv) |R31| greater than |R32|;
(v) T3/F0xe2x89xa60.13 (or xe2x89xa60.1);
(vi) T23xe2x89xa6T34;
(vii) T23xe2x89xa70.15 F0;
(viii) R11 greater than 0;
(ix) R12 greater than 0;
(x) R11 greater than R12;
(xi) F0/F2xe2x89xa70.9;
(xii) F0/F3xe2x89xa60.42 or (xe2x89xa60.4 or xe2x89xa60.3);
(xiii) 0.64xe2x89xa6F0/|F4| less than 0.85 (or 0.75 less than F0/|F4| less than 0.85); and/or
(xiv) the second lens unit comprises a meniscus element which is concave (preferably, strongly concave) to the lens"" long conjugate side.
Characteristics (ii) and (iii) mean that L3 when described in terms of best-fit spherical radii has a meniscus shape convex towards the short conjugate side of the projection lens, while characteristics (viii) and (ix) mean that L1 when described in terms of best-fit spherical radii has a meniscus shape convex towards the long conjugate side of the projection lens.
In certain preferred embodiments of this aspect of the invention, the projection lens has a half angle field of view in the direction of the lens"" long conjugate of at least 35 degrees (e.g., greater than or equal to 37xc2x0 as in Examples 1-8 below). In other preferred embodiments, the projection lens has an f-number for an infinite conjugate of less than 1.5 and preferably xcx9c1.0.
In still further preferred embodiments, the second lens subunit is composed of glass (or, more generally, a thermally stable material) and the first lens element, the third lens element, and the meniscus element of the second lens unit (when used) are composed of plastic. This choice of materials provides a number of advantages to the projection lens.
First, it makes the lens inexpensive to manufacture. As discussed below and illustrated in the examples, the plastic elements of the lens have configurations which allow them to be readily molded in plastic. As also illustrated in the examples, the second lens element (L2) or doublet (DB) used for the second lens subunit has spherical surfaces which allow these components to be readily made in glass. Accordingly, each of the components of the lens can be readily manufactured at low cost.
Second, the plastic-glass-plastic construction of the lenses of the invention, with the glass portion being of strong power, makes the lenses insensitive to changes in temperature. Also, a third lens element (L3) which is composed of plastic and has a positive power provides compensation for changes in temperature of the second lens unit, specifically, changes in temperature of the coupling fluid normally used in that unit and its housing which occur as the unit heats up from room temperature to operating temperature. Again, this thermal stability is achieved for a projection lens that has both a high level of optical performance and a low cost.
In accordance with a third aspect of the invention, the projection lenses of the second aspect of the invention are used in the projection lens systems of the first aspect of the invention.
When the above aspects and preferred characteristics of the invention are used in combination, each of the seven desired features for CRT projection televisions, listed above, are achieved by the projection lens systems/projection lenses of the invention.
As used herein, the term xe2x80x9cweakxe2x80x9d means an element, unit, or subunit whose focal length has a magnitude which is at least about 8 times the effective focal length of the entire projection lens, and the term xe2x80x9cstrongxe2x80x9d means an element, unit, or subunit whose focal length has a magnitude which is less than about 2.5 times the effective focal length of the entire lens system. Also, the effective focal length of the entire projection lens, as well as the effective focal length of the second lens unit, are determined for the projection lens associated with the CRT and includes the optical properties of the CRT""s faceplate.
As used herein, the term xe2x80x9cprojection televisionxe2x80x9d includes televisions and monitors, e.g., computer monitors.
As used herein, the term xe2x80x9cbest-fit spherical radiusxe2x80x9d means the radius determined for a surface by fitting the surface with a best fit sphere in accordance with the procedures described in Dunham et al., xe2x80x9cMinimax Approximation by a Semi-Circle,xe2x80x9d SIAM J. Numer. Anal., 17:63-65, 1980. For a spherical surface, the best-fit spherical radius and the base radius (radius at the optical axis) are identical. For an aspherical surface, the best-fit spherical radius and the base radius will in general be different.